Lithographic printing plate



June 29, 1948. w, G M L EN 2,444,205

LITHOGRAPHIC PRINTING PLATE Filed March 27, 1944 Patented June 29, 1948 I UNITED \sTArEs ATENT OFFICE,

William G. Mullen, Danvers, Mass. Application March 27, 1944, Serial No. 528,257

This invention relates to an improvement in photogravure printing plates and method of making the same.

In accordance with established practice, photogravure printing usually involves the use of either stone, wood or etched metal plates on the one hand, orplates having a printing surface of a sensitized colloidal material such as a hydrophilic protein, on the other hand. The stone, wood and metal plates are expensive and theiruseis generally limited to applications where economic considerations are not of primary importance. Col,- loidal printing plates, although relativelyinexpensive, are not entirely satisfactory in that they are subject to well-recognized disadvantages and limitations, such as are set forth in United States Patent No. 2,179,245, granted November 7, 1939, that is, non-uniformity of coating, variation in sensitivity, rapid deterioration and short life on the press. Although straight casein-coated plates are relatively durable, the removal of casein from the non-printing areas is rather difficult to accomplish and the quality of printing is comparatively inferior. In order to avoid certain of these disadvantages, the use of synthetic hydrophilic resins, of, which polyvinyl alcohol is an example, has been suggested, but this type of plate, although possessing certain advantageous features, nevertheless presents other difiiculties (indicated, for example, in United States Patents Nos. 2,184,- 310 and 2,184,288, granted December 26, 1939), that is, the tendency of the printing areas to lose their ink-receptivity after a relatively short period of use, and their low resistance to solvents.

The principal objects of the present invention are to provide an improved printing plate and 6 Claims. (Cl. 955.4)

applied to a suitable base which is either inherently water-resistant or previously treated sofas not to become weakened or undergo appreciable dimensional changes when subjected to an aqueous treatment. The hydrophilic protein may comprise such materials as refinedgelatim'albumin, soy bean proteinand the like water-dispersi-ble colloids, although for the reasons herein-- after indicated, I preferably use a pigmented casein. t

One or more fillers such as clay, satin white, titanium dioxide, barium sulphate, zincoxide, or the like, may, if desired, be incorporated in the coating solution which isapplied to the base in any conventional manner to produce a smooth uniform coating.

Formalin, aluminum formate or the like insolubilizer may be either incorporated in'the coating solution or subsequently applied to the heating, but only in such arelatively small amount as partially to insolubilize the coating, so that in method of making and. using the same which overcome the disadvantages and limitations above referred 'to, without sacrificing the advantageous features of hydrophilic colloid platesyand to provide a printing plate precoated andcut to the desired size, which plate may be readily sensitized preparatory to use by imbibition of a suitable sensitizer. Further objects will be apparent from a consideration of the following description and the accompanying drawings, wherein I i I Fig. l is a fragmentary perspective, on an exaggerated scale, illustrating a planographic printing plate made in accordance with the present invention and Figs. 2, 3' and. 4 are similar views illustrating successive steps in developing the plate for printmg. i l i l In accordance with the present invention a relatively heavy coating of a hydrophilic protein is any case the resultant coating retains its hydrophilic character in that it is water-sorptive and hence capable of swelling, althoughwater-insolu ble to the extent that it does not undergo disintegration or dissolution when subjected to aqueous treatment.

Two relatively thin coatings 0r layersare then applied over the base coating, the intermediate coating, 1, e., the coating next to thebase coating, comprising a blend of cold water-soluble and cold wateri'nsoluble polyvinyl alcohol in proportions which may vary from approximately 2:1 to 1:2, respectively. The outside or top'coating preferably comprises blended polyvinyl alcohol and/or casein, or the like hydrophilic colloid capable of being sensitized by imbibition of a sensitizing solution which; if desired, may be inhibitedagainst dissolving the top and intermediate coatings; The proportions of cold water-solublepolyvinyl alcohol to cold water-insoluble polyvinyl alcohol may, as above noted, be varied throughout a wide range, depending on such factors as themolecular weight of the polymer, the number of hydroxyl groups, etc, but the preferred proportion is, the order of 1:1. In any case the compositionofa both the top or outer coat and the underlying or intermediate coat should be such that after being sensitized, but before exposure, it should be soluble in cold water, but insoluble in a dilute salt solution, e. g.-, a three percent aqueous sodium sulfate solution; butafter exposure,-both coat ings should be insoluble in cold water. Theweight of the top coat may vary, but the intermediate coat should be sufficiently heavy completelydgo separate the top coat from the base coat so that the unexposed areas of the top coat may be cleanly washed off along with the underlying areas of the intermediate coat so as to leave no polyvinyl alcohol in the non-printing areas.

4. The coatings are applied in solution form in any conventional manner, and the top coats thus produced, when photosensitized, differ from the base coating in that they are Water-soluble so long as they remain unexposed, and hence easily removed from the non-printing areas of the base coat. When selected areas of the top coats are exposed in the conventional manner, such areas become water-insoluble and assume a grease-receptive character, and the underlying areas of the protein coating also become insolubilized to some degree due to the penetration of light through the exposed areas of the top and intermediate coats, whereas the unexposed areas of the top and intermediate coats remain sufilciently water-soluble to be readily dissolved or washed off. Accordingly, the actual printing areas are composed of blended polyvinyl alcohol and/or casein which is adherent to the underlying intermediate polyvinyl alcohol coating carried by the base coat.

The coated sheet material may be out to any desired size or shape, either before or after being photosensitized, and the plate thus prepared may then be subjected to exposure with a negative, master copy or the like, the penetration of light rays through the top and intermediate coats into the base coat being effective not only to harden the printing areas, but also to render these areas water-insoluble and grease-receptive, whereas the unexposed areas of the top and intermediate coats remain water;soluble and the underlying protein coat retains its Water-sorptive and grease-repellent character.

After exposure the plate may be developed by applying a thin layer of a greasy substance, such as developing ink, and following this step the entire plate may then be washed or otherwise subjected to aqueous treatment. Although the developing ink adheres to both the exposed and unexposed areas, due to the water-soluble character of the unexposed portions of the top and intermediate coats, they are readily dissolved and hence the greasy film may be washed off, thus leaving the unexposed areas of the base coat which, as above noted, are grease-repellent. After having washed the plate it may then be wet with a suitable priming solution or the like, which causes the unexposed areas of the base coat to swell to a level'above that of the exposed areas, it being understood that relatively little swelling occurs in the exposed areas due to the hardening of the three layers, attributable to the photo-chemical reaction which takes place during exposure. Thearticle thus produced provides an intaglio printing plate in which the grease-receptive printing areas are disposed below the level of the grease-repellent, non-printing areas and hence protected thereby.

Referring to the drawing, which illustrates what is now considered a preferred procedurethe numeral l designates a flexible paper base, such as a heav sheet of rope stock, kraft or the like, impregnated with a formaldehyde resin or the like water-resisting composition. The paper base i is providedwith a suitable :prime coating 2,

such as a, casein-resin composition, upon which is applied a pigmented caseinbase coat 4. The base coat 4 is preferably composed of casein, satin white and a relatively small proportion of aluminum formate, these materials first being uniformly dispersed in water to form a coating compound which may be brushed, spread or roller coated on the primed base I so as to produce a uniform, dry coating having a weight of the order of 0.7 ounce per square yard.

The intermediate and top coats 5 and 6 are then applied, the intermediate coat 5 consisting of a blend of approximately four parts cold water-soluble polyvinyl'alcohol and six parts cold water-insoluble polyvinyl alcohol, applied in the form of an aqueous solution or dispersion on the dried base coating 4 so as to produce a uniform layer or film preferably having a weight of the order-of 0.1 ounce per square yard. The top coating 6 consists of a mixture of equal parts (dry basis) of the aforementioned blended polyvinyl alcohol and casein, dissolved or dispersed in water containing sufficient alcohol, e. g., propanol, to prevent dissolving or otherwise impairing the integrity of the intermediate coat 5. The coatings are advantageously applied by means of an aniline offset printer which produces a kiss impression from a narrow point of contact. thereby insuring minimum disturbance of the underlying coat. Although the weight of the top coating 6 may vary, as previously indicated, a weight of the order of 0.06 ounce per square yard has been found desirable. Both thecoats 5 and 6 and the underlying base coat 4 are rendered photosensitive by imbibition of an aqueous solution of a suitable chromate, such as a mixture of equal parts of a 6% ammonium bichromate so lution and 2-3% sodium sulfate solution, the percentage of sodium sulfate in any case being the minimum amount required to inhibit dissolution of the coatings.

Although the relative weights of the coatings may vary as previously indicated, it is to be understood that in any case the thickness of the top and intermediate coats 5 and 6 should not only be substantially less than that of the base coat, but also besuch as to permit the penetration of light rays into the base coating 4 so that upon exposure the base coat becomes hardened and more water-resistant than the unexposed areas, and hence capable of swelling to a much lesser extent.

The plate thus prepared, and after being cut to proper size, is ready for use in photogravure printing, and to this end a negative or copy of the work to be reproduced is superposed on the outer coating 5 and the assemblage exposed to light in a conventional manner. For the purpose of illustrationyit is assumed that the letter M is to be reproduced from the negative or mast-er copy, and accordingly the areas 4 of the base coat 4, and 5 and 5 of the top coats 5 and 6, which underlie the letter M during exposure, become hardened, as indicated by the double cross-hatching, and hence grease-receptive and water-insoluble, whereas the remaining or unexposed areas of both the base coat 4 and top coats 5 and 6 remain in substantially their original condition.

After exposure a film I of developing ink or the like greasy material is applied over the entire surface, as indicated in Fig. 2, andfollowing this treatment the inked surface is then washed with water. Due to the water-insoluble and grease-receptive character of the areas 4, 5 and 6 and the water-soluble character of the unexposed areas, the latter readily dissolve and the overlying film of developing ink isthus washed away, whereas the hardened .printingareas 6 retain the film of developing ink 1 applied thereto, as illustrated in Fig. 3.

The final step consists in treating the plate so as to cause the unexposed areas of the base coating 4 to swell, and to this end an aqueous priming solution is applied to the surface of the plate, the priming solution being absorbed by the base coating 4 which swells above the printing areas 8*, thus providing an intaglio plate, as illustrated in Fig. 4.

It should be noted that in a plate made in accordance with the present invention the actual printing areas are composed of a casein-blended polyvinyl alcohol top coat 6*, and that the casein component of the top coat does not come in contact with the casein component of the base coat; hence, the casein components of the two coats are, in effect, insulated by the intermediate coat 5* of blended polyvinyl alcohol, and accordingly, the top coat (casein blended with polyvinyl alcohol) may be cleanly washed off, along with the underlying coat of blended polyvinyl alcohol, from the unexposed areas of the base coat. Inasmuch as the successful use of the plate depends to a great extent upon maintaining separable layers, it is necessary to prevent casein-polyvinyl top coat from becoming intermixed with the underlying and base coats; in other words, it is necessary to maintain separate, distinct, adherent layers, each of which possesses and retains its chemical and physical characteristics.

While I have shown and described one desirable embodiment, it is to be understood that this disclosure is for the purpose of illustration, and that various changes and modifications, as well as the substitution of equivalent materials, may be made without departing from the spirit and scope of the invention as set forth in the appended claims.

I claim:

1. A photogravure printing plate comprising a water-resistant base sheet and three superposed coatings including a base coating comprising a relatively heavy, swellable, partially insolubilized,

water-sorptive, photo-sensitizable protein composition bonded to said base sheet, an intermediate coating consisting of a relatively thin watersoluble, photo-sensitizable polyvinyl alcohol adhering to said base coating, and a top coating comprising a water-soluble, photosensitizable protein adhering to said intermediate coating, the top and intermediate coatings being of such thickness and composition as to permit the penetration of light rays into the base coating, and the thickness of the base coating relative to the top and intermediate coatings being such that unexposed areas of the base coating swell to a level above that of exposed areas of the top coating when said plate is subjected to aqueous treatment.

2. A photogravure printing plate as defined in claim 1, wherein said top coating comprises a water-soluble, photo-sensitizable mixture of polyvinyl alcohol and casein.

3. A photogravure printing plate as set forth in claim 1, wherein said intermediate coating consists of a blend of cold water-soluble and cold water-insoluble polyvinyl alcohol.

4. A photogravure printing plate as set forth in claim 1, wherein said base coating comprises a relatively heavy, swellable, partially insolubilized, water sorptive, photo sensitizable pigmented casein.

5. The method of making an intaglio printing plate, which comprises preparing a plate having on one surface a relatively heavy base coating of of the sensitized plate so as to produce water insoluble, grease-receptive printing areas and water soluble, grease repellent, non printing areas, applying to the surface of said top coating a film of greasy material, subjecting the plate to an aqueous treatment effective to wash ofi the greasymaterial from said non-printing areas together with the underlying water-soluble top and intermediate coatings, and subjecting the plate to a further aqueous treatment to effect swelling of the unexposed areas of the base coat to a level above the printing areas of the top coat.

6. The method of making an intaglio printing plate, which comprises preparing a plate having on one surface arelatively heavy base coating of a sensitizable water-insoluble hydrophilic protein composition bonded to a base member, a relatively thin cold water-soluble intermediate coating consisting essentially of polyvinyl alcohol adhering to said base coating and a top coating comprising a cold water-soluble sensitizable protein composition adhering to said intermediate coating, photosensitizing the top, intermediate and base coatings by imbibition of a sensitizing solution inhibited against dissolving said top and intermediate coats, exposing image-defining areas of the sensitized plate so as to produce water-insoluble, grease-receptive printing areas and water-soluble, grease-repellent, non-printing areas, applying to the surface of said top coating a film of greasy material, subjecting the plate to an aqueous treatment effective to wash on the greasy material from said non-printing areas together with the underlying water-soluble top and intermediate coatings, and. subjecting the plate to a further aqueous treatment to eifect swelling of the unexposed areas of the base coat to a level above the printing areas of the top coat.

WILLIAM G. MULLEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 228,579 Whitley June 8, 1880 1,574,378 Dobinson Feb. 23, 1926 1,944,123 Fleishmann et al. Jan. 16, 1934 2,230,981 Toland Feb. 4, 1941 2,301,770 Bassist Nov. 10, 1942 2,311,489 Toland Feb. 16, 1943 2,311,889 Toland Feb. 23, 1943 2,312,852 Toland et al Mar. 2, 1943 2,327,380 Toland et al Aug. 24, 1943 2,344,487 Bassist Mar. 21, 1944 2,345,447 Bassist Mar. 28, 1944 2,366,723 Galley Jan. 9, 1945 2,373,357 Toland et a1 Apr. 10, 1945 FOREIGN PATENTS Number Country Date 3,578 Germany May 10, 1878 

